Metrum Acoustics Octave: A NOS digital filter-less DAC

[swt61]

Interesting. My Buffalo DAC has a BNC input as well as RCA, but is there really much gear out there with BNC connections?

Edited February 10, 2012 by swt61

[Craig Sawyers]

That is the problem in a nutshell. RCA's are ubiquitous.

And when BNC's are fitted, or cables are BNC fitted more often as not they are 50 ohms, not 75. The difference is easily seen - 50's have insulation around the pin and earth, so when they engage the whole connection is encased in insulation. With 75's there is no insulation at all - the pin and socket mate in air.

It is only because I am anal about interfaces that I've gone to the lengths I have in changing things to be impedance matched throughout, and done some supportive measurements.

[skullguise]

This just made me remember about a company that created their own interfaces (not thinking Krell CAST, but something else) between preamp and amp, and chose BNC 75 ohm connectors as a result. Used to read about it, and the positive aural benefits, in Bound For Sound. Forgot the company, though.....they made some big honkin' SS amps.....

EDIT: found it, PBN Audio, they use BNC as optional connectivity, like their Olypia preamp

[Craig Sawyers]

That is really interesting. Erno Borbely went down the same route with his designs and kits before he retired. Enough drive capability from FET output stage to drive from a 50 ohm source down a matched cable and load. I makes the connection almost totally impervious to cables - just high quality 50-ohm cable is needed.

[Pars]

^ I think KG does this (50 ohm matching) between his home components, or at least I recall he mentioned that he had done it. He also had an interconnect design that was sold by a guy on head-fi a few years ago that had something to do with termination and matching. Can't remember what they were called.

[recstar24]

Nitrogens. Eric something.

[deepak]

^ I think KG does this (50 ohm matching) between his home components, or at least I recall he mentioned that he had done it. He also had an interconnect design that was sold by a guy on head-fi a few years ago that had something to do with termination and matching. Can't remember what they were called.

More

[Pars]

^ Thanks!

gee, there are threads on HC I haven't read

[Craig Sawyers]

That is a good thread - thanks for the link deep. Pity eric's TDR measurements are defunct, but the thread is 6 years old after all.

To quote Borbely from AudioXpress 5/02, about his design for the all-fet line amp design:

"My goal was to be able to drive loads

down to about 100Ω in pure Class-A,

also allowing it to be used as a headphone

amp. To be able to drive 100Ω in

Class-A with up to 10V RMS, you need to

have a bias current of 70mA."

So this would definitely be able to drive 50 ohm cable (so 100 ohm total load for 50 ohm cable) at up to a whopping 10V RMS. Note that to handle that the 50 ohm load resistors would have to be at least half watt - but since the typical maximum you would be putting into an amp is 1V, low power resistors are fine - or even surface mount. I've been intending to build this beast for a while, if only to test my belief (which I now realise that KG shares) that matching impedances should render the analogue link cable insensitive.

[kendrab]

The only thing to watch is that the BNC needs to be insulated from the chassis.

I just ordered a Metrum Octave for myself, and want to switch to a proper BNC connector once I get it home.

What brand and model of BNC connector did you use?

[Craig Sawyers]

I used

TE CONNECTIVITY / GREENPAR

1-1478048-0. It is not the all time ultra-wideband BNC, but it handles (measured) 1ns rise times fine, so it is at least 300MHz bandwidth - amply enough. It has the benefit that (a) it fits the chassis hole in the metrum once you remove the BNC and insulating washers, And it is easy to solder up, and it is easy to return the Metrum to stock condition if you need to ever return it for repair, or sell it.

Edited February 14, 2012 by Craig Sawyers

[Pars]

These two would be good as well:

wrench crimp (for RG174 coax)

http://www.newark.com/trompeter-electronics/ubj26-5/rf-coaxial-bnc-bhd-jack-str-75ohm/dp/06C4325?Ntt=ubj26

solder

http://www.newark.com/trompeter-electronics/ubj27/rf-coaxial-bnc-straight-75ohm-solder/dp/96B2669?Ntt=ubj27

both are insulated and rated for 4GHz

[Craig Sawyers]

These two would be good as well:

wrench crimp (for RG174 coax)

http://www.newark.co...C4325?Ntt=ubj26

solder

http://www.newark.co...B2669?Ntt=ubj27

both are insulated and rated for 4GHz

I think that the first, crimped one might squeeze up to 4GHz, but the soldered ones I'm not so certain - if only because of a mistermination impedance of the coax when it is dressed to solder to the pins. I'm using that sort of course - but even with 192kHz sampling rate you only need 250MHz maximum to take up to the tenth harmonic of the serial SPDIF data.

[Pars]

Yeah, the one you are using should be just fine.

[Craig Sawyers]

Metrum DACs - I have a sneaking suspicion that this http://www.ti.com/lit/ds/symlink/dac714.pdf is what they are using. The only thing that it misses on is the supposed 15MHz operation - but that could apply to clock speed with a bit of hyperbole and misinformation thrown in.

[Currawong]

This is what someone suggested to me as being the chip: http://www.ti.com/product/dac8580

Mine arrived today. I think it's time for another DAC-off.

[Craig Sawyers]

This is what someone suggested to me as being the chip: http://www.ti.com/product/dac8580

That looks very plausible indeed. If it is, Cees must have turned off the digital filter entirely, because the lack of said filter is a specific selling point.

Mine arrived today. I think it's time for another DAC-off.

Whoo! Let us know what you think. Just give it plenty of time to warm up; mine stays powered permanently.

[Filburt]

This is what someone suggested to me as being the chip: http://www.ti.com/product/dac8580

Mine arrived today. I think it's time for another DAC-off.

Looks plausible. Could be DAC8581 in the alternative, which doesn't have an internal filter.

Edited February 20, 2012 by Filburt

[Currawong]

Whoo! Let us know what you think. Just give it plenty of time to warm up; mine stays powered permanently.

I usually leave new DACs on for a couple of weeks before taking them seriously, for good measure. I'd describe it as having a pretty intense, in-your-face sound. Was too much for the 009s but is spot-on with the Stacker II as the bass isn't as strong as from my Reference 7. Once I figure out sourcing the parts, I'm going to copy your mods for sure.

I couldn't ship my Parasound DAC and had to refund the buyer, as India doesn't except electronics from Japan apparently, so I'm going to upgrade it for 24/96 and use it with the Stax. Then I'll have 3 complete rigs -- wonderful overkill.

[gurubhai]

I couldn't ship my Parasound DAC and had to refund the buyer, as India doesn't except electronics from Japan apparently, so I'm going to upgrade it for 24/96 and use it with the Stax.

Don't know why they won't accept stuff from Japan, they do it from almost every place else.

Anyways I am looking to move on & the metrum 'Quad' has caught my interest(budget max. $500). I haven't seen much impressions of that however & that makes me jittery.

Edited February 21, 2012 by gurubhai

[Craig Sawyers]

I usually leave new DACs on for a couple of weeks before taking them seriously, for good measure. I'd describe it as having a pretty intense, in-your-face sound. Was too much for the 009s but is spot-on with the Stacker II as the bass isn't as strong as from my Reference 7. Once I figure out sourcing the parts, I'm going to copy your mods for sure.

I'd agree entirely with that sound on initial switch on. 75-ohm mods give a very worthwhile improvement - but the one that really nailed it was removing the Murata pulse transformer and replacing it with the Lundahl LL1572. A good friend, long-time audiophile and one-time dealer, was across at the weekend. He compared the sound to a Koetsu Red, which is just fine by me!

Some Scientific Conversion transformers arrived yesterday. So I'll have a play with those.

[K3cT]

I usually leave new DACs on for a couple of weeks before taking them seriously, for good measure. I'd describe it as having a pretty intense, in-your-face sound. Was too much for the 009s but is spot-on with the Stacker II as the bass isn't as strong as from my Reference 7. Once I figure out sourcing the parts, I'm going to copy your mods for sure.

Does it have a tendency to over-emphasize details? And how's the soundstage compared to the Reference 7.1?

[Currawong]

Does it have a tendency to over-emphasize details? And how's the soundstage compared to the Reference 7.1?

That's a good description of it -- even subtle sounds stand out. Going from the Ref 7 and Parasound is like going from Miiles Davis to Dave Brubeck.

gurubhai: Here's the full list of prohibited goods. First time I'd heard about it.

Edited February 22, 2012 by Currawong

[gurubhai]

^I understand your problem Amos, but remember that the Parasound has my dibs on it.

I'll see if I can arrange one of my friends to visit you in the near future.

[Currawong]

^I understand your problem Amos, but remember that the Parasound has my dibs on it.

I'll see if I can arrange one of my friends to visit you in the near future.

Make sure he brings a big suitcase! If I can get the Octave to be a good step above, the Parasound will end up drawing the short straw most likely anyway. The Octave has the advantage that it's small enough to take it with me when I travel, go to meets etc.